The treatment of mine drainage-impacted waters generates considerable amounts of sludge, which raises several concerns, such as storage and disposal, stability, and potential social and environmental impacts. To alleviate the storage and management costs, as well as to give the mine sludge a second life, recovery and reuse have recently become interesting options. In this review, different recovery and reuse options of sludge originating from active and passive treatment of mine drainage are identified and thoroughly discussed, based on available laboratory and field studies. The most valuable products presently recovered from the mine sludge are the iron oxy-hydroxides (ochre). Other by-products include metals, elemental sulfur, and calcium carbonate. Mine sludge reuse includes the removal of contaminants, such as As, P, dye, and rare earth elements. Mine sludge can also be reused as stabilizer for contaminated soil, as fertilizer in agriculture/horticulture, as substitute material in construction, as cover over tailings for acid mine drainage prevention and control, as material to sequester carbon dioxide, and in cement and pigment industries. The review also stresses out some of the current challenges and research needs. Finally, in order to move forward, studies are needed to better estimate the contribution of sludge recovery/reuse to the overall costs of mine water treatment.

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Peat and mine drainage treatment sludge can be valorized as amendments on mine sites to stabilize gold mine tailings and reduce the potential leaching of contaminants in pore water. However, the influence of organic amendments on the mobility of metalloids and/or metals in the tailings must be validated, as the leached contaminants may vary according to their type, nature, and origin. The objective of the present study was to evaluate over time the effect of peat‐ and/or Fe‐rich sludge amendments on the mobility of As and metallic cations in the drainage water of tailings potentially producing contaminated neutral drainage. Ten duplicated weathering cell experiments containing tailings alone or amended with peat and/or Fe‐rich sludge (5–10% dry weight) were performed and monitored for 112 d. The results showed that as low as 5% peat amendment would promote As mobility in tailings’ pore water, with As concentrations exceeding Quebec discharge criteria (>0.2 mg L−1). In addition, As(III), the most mobile and toxic form, was predominant with 10% peat, whereas organic species were negligible in all cells. The use of peat alone as organic amendment for the stabilization of tailing contaminants could increase the risk of generating As‐rich contaminated neutral drainage. Conversely, the mix of only 5% Fe‐rich sludge with or without peat decreased As concentrations in leachates by 65 to 80%. Further studies on the use of “peat” or “peat + Fe‐rich sludge” as cover or amendment should be conducted with a focus on Fe/As and Ca/As ratios. Core Ideas Peat amendments enhanced the leaching of As from gold mine tailings. Amendments of 5% peat promoted As(V) leaching, whereas 10% peat increased As(III) leaching. As(III) was predominant at ≥20 mg L−1 dissolved organic C from peat. Mine drainage treatment sludge could decrease As concentrations by 65 to 80% in tailings’ pore water.

a b s t r a c tOrganic amendments aided with vegetation are considered a promising prevention approach for the stabilization of metal-contaminated tailings. However, the inconsistency in the performance of different amendments and their effects on plants growth indicate that further studies are still required. To better select the appropriate organic amendments and identify future research avenues for mine tailings stabilization, this critical review makes a preliminary compilation of available knowledge on organic amendments and their reported performance. To this end, data were collected from several review papers and case studies, with a particular focus on the evolution of pore water quality and plant phytostabilization abilities. The screening of the most promising materials was then carried out according to whether metallic elements were mobilized/immobilized from pore water, sequestered into rhizosphere or plant aboveground parts. Results showed that mixture of organic and inorganic materials are more efficient than organics alone. Amendments combining mature and composted animal manures with inorganic materials would be more promising. Conversely, fresh compost and biosolids could enhance metals release in pore water and, possibly, the transfer in plants aerial parts. Finally, biochars could be efficient if mixed with raw organic amendments but the effect on vegetation still needs to be evaluated. Further studies should focus on amendment-plant-microbes interactions and the long-term stability of organic amendments (>10-20 years).

Multi-step passive systems for the treatment of iron-rich acid mine drainage (Fe-rich AMD) perform satisfactorily at the laboratory scale. However, their field-scale application has revealed dissimilarities in performance, particularly with respect to hydraulic parameters. In this study, the assessment of factors potentially responsible for the variations in performance of laboratory and field-scale multi-step systems was undertaken. Three laboratory multi-step treatment scenarios, involving a combination of dispersed alkaline substrate (DAS) units, anoxic dolomitic drains, and passive biochemical reactors (PBRs), were set up in 10.7-L columns. The field-scale treatment consisted of two PBRs separated by a wood ash (WA) reactor. The parameters identified as possibly influencing the performances of the laboratory and field-scale experiments were the following: AMD chemistry (electrical conductivity and Fe and SO concentrations), flow rate (Q), and saturated hydraulic conductivity (k). Based on these findings, the design of an efficient passive multi-step treatment system is suggested to consider the following: (1) Fe pretreatment, using materials with high k and low HRT. If a PBR is to be used, the Fe load should be < 26 g/m substrate/day (Fe < 200 mg/L) and SO < 110 g/m substrate/day; (2) PBR/DAS filled with a mixture with at least 20% of neutralizing agent; (3) include Q and k (> 10 cm/s) in the long-term prediction. Finally, mesocosm testing is strongly recommended prior to construction of full-scale systems for the treatment of Fe-rich AMD.

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